Metabolism

Definition and Overview

Metabolism refers to the totality of an organism’s chemical reactions. These reactions are essential for life, enabling cells to grow, reproduce, maintain their structures, and respond to environmental changes.

• Metabolism: The sum of all chemical reactions occurring within a living organism.

• Metabolic processes are highly coordinated and regulated, allowing cells to adapt to subtle changes in their environment.

• Metabolism is considered an emergent property of life, arising from the interactions between molecules within the cell.

Overview: The Energy of Life

Cellular Chemical Reactions

Cells function as miniature chemical factories, carrying out thousands of reactions that assemble small molecules into polymers and break them down as needed. These reactions are organized into metabolic pathways.

• Cells can export chemical products to other cells in multicellular organisms.

• Cellular respiration is a key metabolic process that extracts energy from sugars and other fuels to perform work.

• Metabolic pathways are complex, efficient, and responsive to changes in the cell’s environment.

Laws of Thermodynamics

Basic Principles

Metabolic reactions are governed by the laws of thermodynamics, which describe how energy is transferred and transformed in biological systems.

• First Law (Law of Conservation of Energy): Energy cannot be created or destroyed, only transformed from one form to another. The total energy in a closed system remains constant.

• Second Law: Every energy transfer increases the entropy (disorder) of the universe. Spontaneous processes increase entropy, and some energy is always lost as heat.

• Third Law: As a system approaches absolute zero, its entropy approaches a constant minimum.

• Thermal Equilibrium: If two systems are each in thermal equilibrium with a third, they are in equilibrium with each other.

Metabolic Pathways

Types and Functions

Metabolic pathways are sequences of chemical reactions, each catalyzed by a specific enzyme. They can be classified as catabolic or anabolic.

• Catabolic pathways: Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration).

• Anabolic pathways: Consume energy to build complex molecules from simpler ones (e.g., protein synthesis from amino acids).

• Bioenergetics: The study of how organisms manage their energy resources.

Forms of Energy

Types and Transformations

Energy is the capacity to cause change and exists in various forms, some of which can perform work in biological systems.

• Kinetic energy: Energy associated with motion.

• Thermal energy: Kinetic energy due to the random movement of atoms or molecules.

• Potential energy: Energy that matter possesses due to its location or structure.

• Chemical energy: A form of potential energy available for release in a chemical reaction.

• Energy can be converted from one form to another but cannot be created or destroyed.

Thermodynamics in Biology

Energy Transformations and Biological Order

Biological systems obey the laws of thermodynamics. Energy transformations are essential for life but are always accompanied by an increase in entropy.

• Cells convert organized forms of energy to heat, increasing the entropy of their surroundings.

• Spontaneous processes occur without energy input and increase the entropy of the universe.

• Order can increase locally (within cells), but the overall trend is toward greater disorder (entropy) in the universe.

Free Energy and Spontaneity

Gibbs Free Energy

The concept of free energy (Gibbs free energy, G) is used to determine whether a process will occur spontaneously.

• Free energy (G): The portion of a system’s energy that can perform work when temperature and pressure are uniform.

• The change in free energy () during a process is given by:

• = change in enthalpy (total energy)

• = change in entropy

• = temperature in Kelvin ()

• Processes with a negative are spontaneous; those with positive or zero $\Delta G$ are not.

Energy, Stability, and Equilibrium

Relationship Between Free Energy and Stability

Free energy is a measure of a system’s instability. Systems tend to change toward a more stable state (lower free energy).

• During spontaneous change, free energy decreases and the stability of a system increases.

• Equilibrium is a state of maximum stability; metabolic processes in living cells are maintained far from equilibrium.

ATP and Energy Coupling

Role of ATP in Cellular Work

Adenosine triphosphate (ATP) is the cell’s main energy currency, coupling exergonic (energy-releasing) and endergonic (energy-consuming) reactions.

• ATP powers three main types of cellular work: chemical, transport, and mechanical.

• Energy coupling in cells is mediated by ATP, which transfers a phosphate group to other molecules (phosphorylation), making them more reactive.

• The recipient molecule is called a phosphorylated intermediate.

ATP Cycle

ATP is a renewable resource, regenerated by adding a phosphate group to adenosine diphosphate (ADP). The energy for this process comes from catabolic reactions in the cell.

• The ATP cycle transfers energy from catabolic to anabolic pathways.

Enzymes and Activation Energy

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy barrier ().

• Enzymes do not affect the change in free energy () of a reaction; they only increase the rate at which equilibrium is reached.

• Enzymes are highly selective, determining which chemical processes occur in the cell.

Mechanism of Enzyme Action

• The substrate binds to the enzyme’s active site, forming an enzyme-substrate complex.

• The active site lowers the activation energy by:

  • Orienting substrates correctly

  • Straining substrate bonds

  • Providing a favorable microenvironment

  • Covalently bonding to the substrate (in some cases)

Factors Affecting Enzyme Activity

Environmental and Chemical Influences

Enzyme activity is influenced by several factors, including temperature, pH, and the presence of cofactors or inhibitors.

• Each enzyme has an optimal temperature and pH for maximal activity.

• Cofactors: Nonprotein helpers required for enzyme function. Inorganic cofactors include zinc, iron, and copper; organic cofactors are called coenzymes (often vitamins).

Enzyme Inhibitors

Types and Effects

• Competitive inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a conformational change that reduces activity.

• Examples of inhibitors include toxins, poisons, pesticides, and antibiotics.

Regulation of Enzyme Activity

Allosteric Regulation and Feedback Inhibition

Cells regulate metabolic pathways by controlling enzyme activity through various mechanisms.

• Allosteric regulation: A regulatory molecule binds to a site other than the active site, affecting enzyme function. Can either inhibit or stimulate activity.

• Allosterically regulated enzymes often have multiple subunits and active/inactive forms.

• Cooperativity: A form of allosteric regulation where binding of a substrate to one active site increases activity at other sites.

• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

Localization of Enzymes Within the Cell

Compartmentalization

Enzymes are often localized within specific organelles or cellular structures, allowing for regulation and efficiency of metabolic pathways.

• For example, enzymes for cellular respiration are located in the mitochondria.

• Compartmentalization helps coordinate and control metabolic processes.